Gaps and steps between trains and platforms prevent independent usage of trains and slow down trains when they're used for boarding. Ramps are slow, unreliable to those forced to used them, can be unsafe to both users and staff to deploy.  The ramps are often used at stations where the ramp is too steep to meet unassisted disability grades of incline. Ramps can't be made longer due to their weight. Several prototypes of automated ramps have been developed but almost all of them are limited in some way or another. Many fall short of broad deployment and it's often not until cities and states are adopting brand new stations and trains where new trains and new stations ensure a gap free level access experience that the problem is eliminated.